Big-endian compatibility failures in bucket hashing and Avro/deletion-vector serialization #1000

Summary

Junwang (@zhjwpku) raised the big-endian question in the review of #998, which prompted this investigation.

An endian audit of main at 44e29e109e461e7c9069855c9a90442002580d23 found four compatibility problems on big-endian systems:

1. Bucket hashes differ from Iceberg's specified values.
2. Both Avro decimal writers produce incorrect big-endian decimal bytes.
3. The configured Avro dependency uses native byte order for float/double serialization.
4. The configured CRoaring dependency's portable serialization is little-endian-only, affecting deletion-vector bitmap payloads.

These are pre-existing findings, independent of #998. Representative failures were reproduced by executing actual s390x big-endian binaries under QEMU. This was a focused reproduction, not a full Iceberg build or integration test.

Verification scope

• Ubuntu 22.04.5 ARM64 container on a Mac; QEMU 6.2.0; s390x GCC/G++ 11.4.0; statically linked target binaries.
• ELF headers identify IBM S/390, big-endian, and the harness checks native byte order at runtime. No physical big-endian hardware was used.
• Native little-endian control tests pass.
• Murmur: complete, unchanged Iceberg source; blob 14cedabf4ac7b4ece349b52d05e254b86d2a5e8d matches upstream.
• Decimal: actual Arrow 25.0.0 BasicDecimal128 storage plus the original statements from both Iceberg writers. Full Arrow arrays and Iceberg writers were not instantiated.
• Avro: four unchanged float/double routine bodies from pinned commit 209a3735ec330679790824da54b7558db55e4e7f, using byte-copy stream substitutes. The full Avro library was not linked.
• CRoaring: complete, unchanged 4.4.3 library and its original C++ wrapper, with no stubs.

Dependency findings below apply to the configured bundled versions. Custom dependency versions may differ.

1. Bucket hashing reads little-endian input as native words

getblock32 and getblock64 return p[i] without endian conversion. The x86_32 block loop uses these native loads. Although HashLong prepares little-endian input, the block reads interpret it incorrectly on a big-endian host.

Observed signed 32-bit hashes and bucket[100] results:

The UUID golden vector also fails; a three-byte tail-only control passes. The failure is consistent with native full-block loads, rather than the byte-based tail handling.

This affects bucket transforms for numeric/date/time/timestamp/UUID inputs and byte/string inputs with full four-byte blocks. The same underlying hash code also affects object-store path hashing, with a lower interoperability impact.

Suggested direction: explicit little-endian, alignment-safe block loads, consistent with Iceberg's Appendix B 32-bit hash definition. Merely converting the input buffer to little-endian does not fix the subsequent native loads.

2. Avro decimal writers unconditionally reverse native bytes

Both direct encoder and data utility writer reverse the native decimal bytes unconditionally.

For a 16-byte decimal with unscaled value 1:

Expected big-endian wire: 00000000000000000000000000000001
Actual s390x wire:       01000000000000000000000000000000
Actual interpreted value: 2^120

On big-endian Arrow storage, the reversal corrupts the intended big-endian wire representation. Suggested direction: reverse only on little-endian hosts, or use an explicit big-endian conversion. The reader-side nanoarrow decimal copy is not the source of this failure.

3. Pinned Avro float/double routines use native wire order

The configured Avro revision's encoder and decoder copy native floating-point bytes, while Avro requires little-endian wire order.

Same-host encode/decode round trips pass, hiding the cross-endian incompatibility. Both Iceberg Avro backends use the affected dependency. Suggested direction: fix/update the dependency or explicitly reject unsupported big-endian configurations until this is resolved.

4. CRoaring portable serialization is incompatible on big-endian hosts

Iceberg writes and reads the inner bitmap through CRoaring's portable serialization APIs.

With the complete CRoaring 4.4.3 library and bitmap {1}, the s390x execution:

• emits bytes different from the valid little-endian portable fixture;
• rejects that valid little-endian fixture when reading;
• successfully round-trips its own output.

CRoaring's 4.4.3 README explicitly limits serialization support to little-endian systems. Its writer and reader use native byte order.

Iceberg's outer deletion-vector header endian handling is correct; the inner serialized bitmap remains incompatible. Suggested direction: guard big-endian use of this path until the dependency provides compatible serialization.

Regression coverage

Cross-endian golden fixtures are needed in addition to same-host round trips: bucket hash vectors, decimal wire bytes, Avro float/double fixtures, and deletion-vector bitmap fixtures. A full Iceberg integration run on a big-endian target remains untested.